Uniformity of polyester films

ABSTRACT

CONVENTIONALLY HEAT-SET, BIAXIALLY ORIENTED POLYESTER FILMS SUCH AS POLY(ETHYLENE TEREPHTHALATE) GENERALLY HAVE NON-UNIFORM PHYSICAL PROPERTIES ACROSS THE WIDTH OF THE FILM. IT HAS BEEN DISCOVERED THAT THE RELATIVE UNIFORMITY OF SUCH PROPERTIES ACROSS THE WIDTH OF THE FILM CAN BE SIGNIFICANTLY INCREASED BY STRETCHING THE FILM IN THE TRANSVERSE DIRECTION (DURING THE TENTERING STEP) WHILE THE SURFACES OF THE FILM ARE MAINTAINED AT A SUBSTANTIALLY UNIFORM TEMPERATURE OF AT LEAST ABOUT 260*F., WHICH IS HIGHER THAN WAS HERETOFORE BELIEVED PRACTICAL IN A TENTERING OPERATION.

United States Patent 3,702,357 UNIFORMITY OF POLYESTER FILMS Edward W.Smith, Jr., Rochester, N.Y., assignor to Eastman Kodak Company,Rochester, N.Y.

No Drawing. Continuation of abandoned application Ser. No. 820,662, Apr.30, 1969. This application June 25, 1971, Ser. No. 157,045

Int. Cl. B29c 17/02, 25/00 US. Cl. 264-289 3 Claims ABSTRACT OF THEDISCLOSURE This application is a continuation of Ser. No. 820,662 filedApr. 30, 1969, now abandoned.

This invention relates to improved polyester films having not only theexcellent dimensional stability of conventional biaxially oriented,heat-set (thermally stabilized) polyester films, but also having a moreuniform distribution of physical properties across the width of thefilm.

The desirability of manufacturing films having substantially uniformphysical properties in both the length and width directions at any pointacross the width of the film is well recognized. In the case ofpolyester fihns, a typical example of somewhat undesirable, non-uniformproperties is the variation in tensile break strength that exists acrossfilms that have been biaxially oriented and heat set via conventionalcommercial processes for their manufacture. Thus, the tensile breakstrength shown at one point across the width'of a typical conventionalbiaxially oriented, heat-set polyester film (for example, at a pointnear one edge of the film) might differ by a factor of as much as 30% ormore from that of the same film when tensile break strength is measuredat a point near the center of the film. More uniformity of the variousphysical properties (such as, for example, tensile break strength,tensile yield, modulus of elasticity, and thermal and humiditycoefficient of expansion) of the film across its width is necessary whenit is desired to produce films having optimum properties for any of anumber of uses, such as a base or support for highest qualityphotographic films, highest quality engineering drafting sheets, and thelike.

Unfortunately conventional processes involving the steps of continuouslystretching, tentering, and heat-set ting long strips of polyester filmsinvariably results in the production of films having a significantamount of variation (across their widths) in many of their importantphysical properties. Films that have such significant variations inphysical properties across their width have also been found to have afairly high birefringence. Birefringence has been found to be somewhatproportional and directly correlatable with the degree of non- "iceuniformity of the various physical properties across the width of thepolyester films.

The term birefringence refers to the dilference in refractive indiciesbetween the major and the minor directions in the plane of the film.Thus, a birefringence value of 0 indicates that the sheet of film hasuniformly balanced properties in all directions in the plane of thesheet at the point at which the birefringence is measured.Theoretically, optimum biaxially oriented, heat-set polyester filmsshould have their major axis parallel to either the length or width ofthe film. The minor axis is always perpendicular to the major axis, ineither case. Also, for optimum uniformity, the extent or amount ofbirefringence must be essentially very low across the entire width ofthe film. Thus, if the direction of the major axis is exactly parallelto the width (or length) of the film in the center of the film, foroptimum stability and uniformity, the direction of the major axis shouldbe parallel to the width (or length) of the film at every point acrossthe width of the film and the birefringence should be very low at eachpoint across the film, too. Birefringence values would be expected toamount to as much as 0.020 or more in conventionally commerciallyproduced poly(ethylene terephthalatc) films, for example.

It has now been discovered that the relatively high birefringence valuesdescribed above can be reduced substantially or even practicallyeliminated, provided that the polyester film is subjected to anunusually high temperature during that stage or step of the overallmanufacturing process in which the film is stretched laterally ortentered. Actually the temperatures of the film surfaces must be atleast about 260 F., and up to about 330 091, during at least about ofthe time in which the film is being tentered. As a practical matter thetemperature of the film surfaces cannot be within the essential region(that is, above about 260 F.) during the entire tentering step becauseof the amount of time necessary to raise the temperature of the filmsurfaces into the essential region from temperatures below about 250 F.(at which the filrn should be preheated prior to the tentering step).For some apparently unexplainable reason, for the successful practice ofthis invention, the essential (film surface) temperatures must beattained simultaneously with at least the initial stages of thetransverse stretching or tentering step. Thus, it has been theexperience heretofore that the use of such very high film temperaturelevels prior to the actual tentering operation (for example, in apreheat section of a conventional polyester film machine) resulted inincreased birefringence values, apparently due to an undesirable degreeor type of crystallization occurring in the film during such a hightemperature preheat step.

As it was indicated hereinbefore, the present processes can be used tocorrect the undesirable variation in valuable physical properties acrossthe width of the film described hereinbefore no matter what the detailedprevious history of the film had been, so long as the film is a mainlylinear polyester and can be biaxially oriented. Such polyester films aremade commercially, for example, via processes such as those described inU.S. latent 2,779,684 and in US. Patent 2,823,421. Any polyester thancan be manufactured in the form of a film and that can be biaxiallyoriented and heat-set in accordance with such processes as those ofthese patents can be treated successfully in the practice of the presentinvention. However, in the pre ferrcd practice of this invention, thefilms that are treated arethose comprising poly(ethylene terephthalate)or polygenerally significantly better properties than that of the(cyclohexylene-l,4-dimethylene terephthalate). conventionally preparedcomparison;

TABLE I Maximum shrinkage Young's modulus Break elongation differentialat differential, p.s.i. differential, percent approx. 173 C.,perccntBirefringence (major-minor) (major-minor) (major-minor) Stretch airtemp, F. Avg. Edge Center Edge Center Edge Center Edge Center Generally,in processes for manufacturing relatively thin The invention has beendescribed in detail with particufilms of oriented polyesters, it hasbeen found that, when lar reference to preferred embodiments thereof,but it will films are introduced into a relatively hot zone, the surface15 be understood that variations and modifications can be temperature ofthe film rises fairly quickly to within about effected within the spiritand scope of the invention. 15 F. of the temperature of the airsurrounding the film I claim: in the hot zone, although the temperatureof the air gen- 1. In a process for manufacturing biaxially orientederally remains slightly higher than that of the film surfaces, polyesterfilm in which the polyester is comprised of particularly When hot airconstitutes the main source of repeating residues of glycol anddicarboxylic acid or ester heat for the film. Thus, in the tenteringstep of the present units, which process comprises biaxially orientingsaid processes, when it is desired to raise the temperature of the filmby stretching it longitudinally and transversely at an film surfacesinto the temperature region in which the elevated temperature; theimprovement which comprises above-described benefits of this inventioncan be obtained, raising the temperature of the surface of the filmwithin it is generally preferred to maintain the temperature of the therange of from about 260 F. to about 330 F. substanair in the tenteringzone Within the range of from about tially simultaneously with at leastthe initial stages of the 275 F. to about 340 F. Then, the temperatureof the transverse stretching, and maintaining the temperature filmsurfaces increases quickly to the effective range after substantiallyuniform across the surface of the film within the film is introducedinto this region that contains very said range during at least about 80%of the time in which a hot gas. Thus, the effective film surfacetemperature is said film is being stretched transversely, saidtemperature attained Within a very short time of that point in time atof the surface of the film being below the melting temwhich thetentering operation is begun (when the film is perature of thepolyester.

introduced into the relatively hotter tenter zone), and is 2. Animproved process as in claim 1, wherein said polymaintained though atleast about 80% of the tentering ester is poly(ethylene terephthalate).

step. 3. An improved process as in claim 1, wherein said poly- EXAMPLEester is poly(cyclohexylene 1,4-dimethylene terephthalate).

In the production of a nominal 4 mil film of poly(ethyl- ReferencesCited ene terephthalate) on a conventional, commercial UNITED STATESPATENTS machine, the air temperature utilized in the tenter was 40maintained at approximately 310 F. while all other oper- 2779684 1/1957Anes 264-289 ating parameters were held essentially constant. A com-2823421 2,1958 Scarlett 264-289 parison (conventional production run wasmade utilizing 3161711 12/1954 Tassler 264-289 an air temperature in thetenter of 210 F. The birefring- 3 A29 961 2/1969 Spencer 264' 289 enceof the film made via the conventional procedure at FOREIGN PATENTS 210F. had an average value across a 4 foot width of the 235,432 9/1961Australia 264 289 film of 0.01 6, while the birefringence of the filmmanufacture in accordance with the present invention (using a ROBERT F.WHITE, Primary Examiner tenter air temperature of 310 -F.) had anaverage birethe table below are listed some comparative data resultingfringence across the film of less than half that (0.007). In THURLOWAssistant Examiner from several physical tests performed on theresulting films. Note that the film made via the present process has 260R; 264-210 R, 235, 290 T, 346

